This invention relates generally to optical memory techniques and devices, and, more particularly, relates to optical data storage techniques and devices utilizing holographic storage in volume media, in conjunction with coherent writing and reading beams.
In recent years, a wide range of different optical media have been considered or proposed for providing high capacity data storage and retrieval. The dimensionality of various media is one identifying characteristic of optical storage systems. Two-dimensional media, such as optical disk and microfiche storage devices, are common. Three-dimensional memory media are also possible. Three-dimensional media include, for example, volume holographic memories. Such media are discussed in R. J. Collier, C. B. Burckhardt, L. H. Lin, "Optical Holography" Academic Press, New York (1971) pp. 454-493, incorporated herein by reference.
A second identifiable characteristic is the use of either holographic or bit-oriented storage. Although schemes have been proposed for selective erasure of data in holographic memories, selective erasure using bit-oriented storage is conceptually simpler. Holographic storage methods in which the reading or writing radiation is incident on the entire memory medium are limited in information capacity by the erasure of old data during reading operations or the storage of new data. However, photon gating or electronic gating of memory planes can solve these problems for both holographic and bit-oriented storage. Holographic storage is less susceptible to dust and media imperfections, but the same effect may be achieved using bit-oriented storage through the use of coding schemes. Holography provides a method of storing and accessing information stored throughout the volume of a medium without the requirement for a complex optical system to access individual planes in the medium.
It is known that multiple two-dimensional planes of data can be stored in a volume holographic medium, and that these planes may be accessed individually by introducing the reference beam into the medium at a different angle for each individual plane of data. A volume medium therefore has three spatial dimensions, corresponding to the dimensionality of the information stored in a two-dimensional output array multiplied by the number of independent reference beam directions in a linear array of reference beams.
Certain conventional optical data storage systems, such as optical disk memory, can provide large storage capacity. In particular, 30,000 tracks multiplied by 150,000 bits per track results in a capacity of approximately 560 Mbytes on a 12 cm optical disk. However, the use of a single detector for readout provides only a serial data stream, which limits the data transfer rate. The disadvantages of this memory device also include difficulties caused by the dynamic focusing and tracking problems associated with a moving disk, the latency or time required to wait for the desired bits to rotate to the reading location, and the wide field of view lens required for bit-oriented access.
In other optical data storage systems, cascaded orthogonal beamsteering stages are used to access data pages stored in a two-dimensional holographic format. In this system, the memory medium is stationary, eliminating the need for active tracking in the beamsteering system. Because the data are stored holographically, no readout optics are required, eliminating the need for a wide-field-of-view high-resolution lens. Parallel readout can be used to obtain an entire two-dimensional array of bits from one beam position, allowing the use of a somewhat slower beamsteering mechanism to be used than for an optical disk, while still maintaining the same data transfer rate. A millisecond deflection time provides a possible data rate of 10.sup.9 bits per second, which exceeds the data transfer rates of current detector arrays.
However, two-dimensional holographic memory requires high spatial frequency response, and is characterized by limited storage capacity and excessive size, because the information is spread out over a two-dimensional area. The storage capacity of two-dimensional holographic memory is limited by the resolution of the medium. Assuming an array of 1000 by 1000 bits is stored in a 1 cm by 1 cm hologram, a 10 cm by 10 cm memory plane can contain 100 holograms with 10.sup.6 bits per hologram or 10.sup.8 bits in total. Since each bit is about 10 .mu.m in size at the detector array, the optical system must have an optical configuration of approximately f/20 for a 0.5 .mu.m reading wavelength. Thus, the detector must be approximately 20 cm from the memory plane. Since none of the pages can be directly on-axis, the pages at the far side of the 10 cm by 10 cm array must have an angle of approximately 60.degree. between the illumination and the signal beam, corresponding to a hologram fringe spacing of one wavelength, or 0.5 .mu.m. Storage of more holograms in a single memory plane would require even greater spatial resolution.
There has long been a need for a high capacity optical data storage technique which eliminates the requirements for mechanical translation or rotation of a storage medium and read/write element--with its associated latency and tracking problems--while providing compact, high density data storage.
It is accordingly an object of the invention to provide a high capacity optical data storage technique in which both the storage medium and the read/write element are substantially stationary, and which provides high access speeds.
It is another object of the invention to provide methods and apparatus adapted for interconnection computing systems.